Finally, portage supports hooks for executing abirtray functions during an emerge process. This is done through /etc/portage/bashrc and some knowledge about ebulid(1). I have created this little sample that finally frees me from making a backup of every going-to-be-unmerged package.
# file /etc/portage/bashrc
if [[ ${EBUILD_PHASE} == "setup" ]]; then
einfo “Running quickpkg…”
/usr/sbin/quickpkg `echo ${EBUILD} | rev | cut -f2 -d/ | rev`
fi